Optical fibre sensor systems have been deployed in many different applications due to their general distributed nature. Such systems enable that measurements may be made either distributed, or quasi-distributed, i.e. in a potentially large number of points along the fibre. Furthermore, optical fibre sensors in general lend themselves to being multiplexed along a single fibre, thus potentially reducing number of fibres and complexity and bulky lead-in cables, etc.
One class of quasi-distributed optical fibre sensor systems is based on series of reflectors arranged along one or more fibres in an array. Reflectors may, e.g., be fibre Bragg gratings (FBGs), splices, or other types of fibre perturbations resulting in a reflectance larger than an intrinsic backscatter level along the fibre. Reflected signals from the reflectors may for instance be used in interferometric sensor arrangements to deduce the distance, or the variation in distance to the reflectors, or between sets of reflectors.
Interferometric sensor arrays based on FBG reflectors along a sensor fibre has typically been interrogated with wavelength division multiplexing (WDM), having FBGs at different wavelengths, and/or time division multiplexing (TDM), using pulsed interrogation to interrogate an array of equal wavelength FBG based interferometers, as described in U.S. Pat. No. 7,366,055 by the same application, which is hereby incorporated by reference.
Increasing the number of time-multiplexed sensors along the same fibre will typically require a reduced pulse duty cycle, scaling inversely proportional to the number of sensors. This will reduced the dynamic range and the sensor phase resolution due to lower time averaged optical power at the receiver.
One way to obtain distributed sensing with a very large number of interferometric sensor sections along a single fibre is to exploit the Rayleigh back-scattering in the fibre, as e.g. described in U.S. Pat. No. 7,764,363, using pulsed interrogation which separates the sensor sections along the fibre in time.
An alternative interrogation technique is to use the coherent optical frequency domain reflectometry (C-OFDR)-method, which is commonly used for high spatial resolution characterisation of attenuation and backscattering in optical fibres and components. Here, frequency swept laser light is launched into the fibre under test and the return light is coherently detected by mixing the reflected light with a reference signal at a receiver. Thus, light backscattered from different longitudinal positions will be separated in the frequency domain at the receiver.
US 2012/0174677 A1 (Hill) discloses an C-OFDR-based sensor system for distributed interferometic sensing based on Rayleigh scattering along a fibre for measurements of mechanical parameters, in particular mechanical vibrations along the fibre.
Distributed interferometric sensing based on the Rayleigh scattering of the fibre provide limited sensor phase resolution due to the very low optical power levels reflected back to the receiver.
Hence, an improved optical fibre sensor system would be advantageous, and in particular a more efficient and/or improved method of interrogating an optical fibre sensor system would be advantageous.